What you'll learn
- How visual information travels from the eye to the brain.
- Why early visual experience affects development of the visual cortex.
- What animal studies have shown about critical periods and brain plasticity.
- How habituation can be investigated and distinguished from fatigue or sensory adaptation.
Starting points: neurons, stimuli and learning
A stimulus is a detectable change in the internal or external environment. In vision, the stimulus is light. A receptor detects a stimulus and converts it into electrical signals in the nervous system. These signals are carried by neurones, specialised cells that transmit impulses called action potentials.
Neurones communicate at synapses, which are junctions where one neurone influences another, usually by releasing chemicals called neurotransmitters. Learning involves a change in behaviour or response as a result of experience, and in the nervous system this usually means changes in synapses or in the strength of neural pathways.
Plasticity
Plasticity is the ability of the nervous system to change its structure or function in response to experience, activity, injury or development.
The visual pathway: from eye to visual cortex
The retina is the light-sensitive layer at the back of the eye. It contains photoreceptors, cells that detect light and begin the process of converting it into nerve impulses. The information leaves each eye through the optic nerve.
The visual field is the part of the outside world seen by the eyes while looking straight ahead. Information from the left visual field is processed mainly by the right side of the brain, and information from the right visual field is processed mainly by the left side. This happens because some nerve fibres cross over at the optic chiasma.
The pathway is:
- Retina
- Optic nerve
- Optic chiasma
- Optic tract
- Lateral geniculate nucleus in the thalamus
- Primary visual cortex in the occipital lobe
Primary visual cortex
The primary visual cortex, also called V1, is the area at the back of the brain that first processes detailed visual information such as edges, orientation, position and movement.
Tracing a left visual field signal
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A point in the left visual field forms images on the nasal retina of the left eye and the temporal retina of the right eye.
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Axons from the nasal retina cross over at the optic chiasma, while axons from the temporal retina stay on the same side.
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Both sets of information therefore travel to the right side of the brain, so damage to the right primary visual cortex can affect vision in the left visual field of both eyes.
Eye side versus visual field side
Do not say “the left eye goes to the right brain”. Each eye sends information to both sides of the brain. It is the left visual field that is mainly processed by the right visual cortex, and vice versa.
Visual development: experience tunes the system
Some parts of the visual system are set up by genes before birth. For example, axons grow from the retina towards particular brain regions. However, the fine details of the system depend strongly on visual experience after birth.
During early life, neurones in the visual cortex form many synaptic connections. Connections that are used frequently are strengthened, while connections that are rarely used may be weakened or removed. This removal of unused connections is called synaptic pruning.
Critical period
A critical period is an early stage of development when the nervous system is especially sensitive to particular experiences. If the right stimulation is missing during this period, normal development may be permanently affected.
The visual cortex contains neurones that respond best to particular features. A receptive field is the area of the visual field that affects the activity of a particular neurone. Some neurones act as feature detectors, responding strongly to edges or bars at a particular angle. Groups of neurones may be arranged into columns, including ocular dominance columns, where neurones respond more strongly to input from one eye than the other.
Use it or lose it
Visual development depends on competition between neural pathways. Active pathways are strengthened; inactive pathways are weakened, especially during the critical period.
What animal studies showed
Scientists used animal studies because it would be unethical to deliberately deprive human babies of normal visual experience. Mammals such as kittens and monkeys have visual systems similar enough to humans to provide useful evidence, although findings must still be applied carefully.
Monocular deprivation
In classic work by Hubel and Wiesel, one eye of a young animal was closed during the critical period. This is called monocular deprivation, meaning deprivation of one eye. When the eye was later reopened, the eye itself could still be structurally normal, but the visual cortex responded poorly to input from that eye.
The explanation is that the open eye’s neural pathways had been active and therefore strengthened. The closed eye’s pathways had been inactive and were outcompeted in the visual cortex.
Explaining monocular deprivation
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During the critical period, the closed eye sends little or no visual input to the visual cortex, while the open eye continues to send frequent input.
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Synapses from the active eye are strengthened because those pathways are repeatedly stimulated; synapses from the inactive eye are weakened or pruned.
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When the closed eye is reopened, light can still enter the eye, but fewer visual cortex neurones respond to it, so vision from that eye is impaired.
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If the same deprivation happens after the critical period, the effect is usually much less severe because the main cortical connections are already established.
Restricted visual environments
Blakemore and Cooper raised kittens in environments containing only vertical or only horizontal stripes. Later, the kittens had difficulty responding to orientations they had not experienced. Recordings from the visual cortex showed fewer neurones responding to the missing orientation.
This supports the idea that the visual cortex needs appropriate visual stimulation to develop normal feature detectors.
How to evaluate animal studies
A strong evaluation usually includes both sides: animal studies allow controlled investigation of brain development that cannot be done in humans, but they raise ethical issues and results may not transfer perfectly between species.
Ethical issues
Many visual development studies involved deprivation, restricted environments or invasive recording from neurones. Modern research must consider the 3Rs:
- Replacement: use non-animal methods where possible.
- Reduction: use the minimum number of animals needed for valid results.
- Refinement: reduce pain, stress and long-term harm.
Habituation: learning to ignore harmless stimuli
Habituation is a simple form of learning where an animal shows a reduced response after repeated exposure to a harmless stimulus. It is non-associative learning, meaning the animal is not learning a link between two different stimuli; it is simply changing its response to one repeated stimulus.
For example, a snail may initially withdraw its eyestalks when touched gently. If the same harmless touch is repeated many times, the withdrawal response may become shorter or smaller.
At the neural level, habituation may involve reduced neurotransmitter release at synapses in the response pathway. The motor response is therefore less likely or less intense.
Dishabituation
Dishabituation is the return of a response after a new stimulus is given, or after a rest period. It helps show that the animal was learning to ignore the repeated stimulus rather than simply being unable to respond.
Habituation is not the same as fatigue
If an animal stops responding because its muscles are exhausted, that is fatigue, not habituation. Evidence for habituation is stronger if the animal responds again to a different stimulus.
Investigating habituation in practice
A typical investigation might use snails and measure withdrawal time after a gentle, standardised touch.
The independent variable is the number of repeated stimulations or trial number. The dependent variable is the response, such as withdrawal duration in seconds. Control variables include the force of the stimulus, the position touched, the time interval between trials, temperature, light intensity and handling.
A good method would:
- Allow the animal to acclimatise to the environment.
- Apply the same harmless stimulus at fixed intervals, for example every 30 s.
- Measure the withdrawal response duration in seconds.
- Repeat for several trials and several individuals.
- Calculate a mean response for each trial number and plot a graph.
Calculating the size of a habituation effect
A snail’s withdrawal response is 12.0 s on trial 1 and 1.2 s on trial 10.
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Choose a calculation that compares the fall in response with the starting response:
percentage reduction=initial response−final responseinitial response×100\text{percentage reduction} = \frac{\text{initial response} - \text{final response}}{\text{initial response}} \times 100percentage reduction=initial responseinitial response−final response×100 -
Substitute the values, carrying the unit seconds through the subtraction:
12.0 s−1.2 s12.0 s×100\frac{12.0\text{ s} - 1.2\text{ s}}{12.0\text{ s}} \times 10012.0 s12.0 s−1.2 s×100 -
Calculate the change in response:
10.8 s12.0 s×100=90.0%\frac{10.8\text{ s}}{12.0\text{ s}} \times 100 = 90.0\%12.0 s10.8 s×100=90.0% -
Interpret the result: the withdrawal response decreased by 90.0%, which is consistent with habituation if the stimulus was harmless and the animal could still respond to a novel stimulus.
Bringing it together
Visual development and habituation both show that the nervous system is not fixed. Experience can change neural pathways. The difference is mainly the timescale and permanence: visual development during a critical period can cause long-lasting structural changes, while habituation is usually a shorter-term and reversible change in response.
In the exam
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When explaining visual deprivation, link missing stimulus, inactive synapses, competition, and reduced cortical response.
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When evaluating animal studies, include both scientific value and ethical limitations; do not give only one side.
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For habituation practicals, always distinguish habituation from fatigue by discussing controls, recovery, or response to a novel stimulus.
Check yourself
- Why can a normal-looking eye still have poor vision after early monocular deprivation?
- How does the optic chiasma explain visual field processing on opposite sides of the brain?
- What evidence would help you decide whether a decreasing snail response is habituation rather than fatigue?
